You may have heard the term "illegal immigrants" in the news or mentioned by adults. What does that mean, exactly? How can a person be illegal?

It's kind of a weird thing to call someone, so we choose to use the term "undocumented immigrants," meaning they don't have the official paperwork that allows them to live in a new country. When immigration through legal channels is too expensive, complicated, or slow, or when the situation is too desperate, people are often left with this option.

Do you picture undocumented immigrants sneaking across national borders in the dark of night or hidden in a truck? This does happen. But just as often, families will enter a new nation as visitors and then choose to stay for good. For many, this new life is better than where they lived before, but it usually has serious challenges and dangers too.

A life of fear. For most undocumented immigrants, fear -- and the stress that comes with it -- is a constant part of life. Adults and kids alike must keep their situation a secret from nearly everyone they meet, afraid they'll be reported and sent back.

A life of invisibility. Many undocumented immigrants work very hard to stay "invisible." They avoid things that will call attention to themselves, or make people ask questions. They feel the need to blend in and disappear, because there's safety in not being noticed.

Being used and abused. Undocumented immigrants are often taken advantage of by the people they work for. Employers might feel free to pay low wages and ignore dangerous conditions, because the workers have no legal way of complaining. If they go to the authorities, they risk being arrested themselves and deported. If they complain or make trouble in other ways, the bosses can call immigration and have them taken away. Landlords can do this too, forcing people to live in terrible conditions because they have no way to complain or fight back.

The perils of going "legal." Many undocumented immigrants want to become citizens of their new countries, or at least get official documents that will allow them to stay and work. But to apply for these documents, they must make themselves known to the authorities. They might feel that going through "legal channels" will have the opposite effect, and that once the government knows about them they'll get sent back to their home country.

Living without. Because undocumented immigrants live in fear of being discovered, they often do without services that others take for granted. Parents might not enroll their kids in school, or take them to the doctor when they're sick, for fear that these things could get them arrested or deported.

When the "home country" isn't home. Undocumented immigrants can live their whole lives in a country that they aren't legally allowed to be in. Take, for example, a family that crosses into the U.S. from Mexico with a two-year-old daughter. By the time that child is an adult, she will be American in almost every way, but still not a citizen. If her immigration status is discovered, she could be sent to Mexico-a country she has no memory of, and no place to live in. She might not even speak Spanish or have any relatives there!

Sometimes, children will grow up unaware that they're not a legal citizen of their country, because their parents never told them their real immigration status. When it comes time for them to apply for college or jobs, they discover the truth-which often throws a big wrench into their plans for the future.

Because of all these challenges, millions of people continue to live and work in countries without proper documentation. It's a complicated issue, and one that people have very strong opinions about. People fight and argue and try to find ways to fix the system. In the meantime, countless immigrants go on living in secrecy and fear, and that includes countless children, tweens, and teens.